#include <bits/stdc++.h>
#include <functional>
using namespace std;

#ifndef ONLINE_JUDGE
#include <bits/debug.h>
#endif

#define int int64_t
#define endl "\n"

using PII = pair<int, int>;
using TII = tuple<int, int, int>;
template <class T> using V = vector<T>;
using i64 = long long;
using i128 = __int128;
using u64 = unsigned long long;

#define FOR(i, a, b) for (int i = (int)(a); i <= (int)(b); i++)
#define FOR2(i, a, b, c) for (int i = (int)(a); i <= (int)(b); i += (int)(c))
#define FORD(i, b, a) for (int i = (int)(a); i >= (int)(b); i--)
#define FORD2(i, b, a, c) for (int i = (int)(a); i <= (int)(b); i -= (int)(c))
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define lowbit(x) ((x) & (-x))
#define RE return;
#define Yes cout << "Yes" << endl;
#define YES cout << "YES" << endl;
#define No cout << "No" << endl;
#define NO cout << "NO" << endl;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define sz(x) (int)(x).size()

void solve() {
    int n;
    cin >> n;
    string s, t;
    cin >> s >> t;

    V<int> p(256 + 1, 0), siz(256 + 1, 0);

    FOR(i, 1, 256) p[i] = i, siz[i] = 1;

    function<int(int)> find = [&](int x) {
        if (x != p[x])
            p[x] = find(p[x]);
        return p[x];
    };

    map<char, char> mp;

    int ans = 0;

    V<int> vis(256, 0);

    FOR(i, 0, n - 1) {
        if (mp[s[i]] && mp[s[i]] != t[i]) {
            cout << -1 << endl;
            RE;
        }
        char x = s[i], y = t[i];
        if (x > y)
            swap(x, y);
        int a = find(x), b = find(y);
        if (a != b) {
            p[b] = a;
            siz[a] += siz[b];
        } else if (!mp[s[i]]) {
            vis[a] = 1;
            if (siz[a] == 26) {
                cout << -1 << endl;
                RE;
            }
            ans += (siz[a] - 1) * 2;
        }
        mp[s[i]] = t[i];
    }

    FOR(i, 'a', 'z') {
        if (vis[i])
            continue;
        ans += (siz[i] - 1);
    }
    cout << ans;
}

signed main() {
    int Task = 1;

    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    while (Task--) {
        solve();
    }

    return 0;
}
